Tissue transplants from one individual to another are at risk of rejection. The degree of risk is proportional to the degree of disparity between certain genetic products, antigens, expressed on the surface of donor and recipient cells. To ensure the success of a transplant, donor tissue must be immunohistologically compatible with recipient tissue, that is, donor and recipient tissue must be matched. Matching is accomplished when donor tissue expresses “self” antigens. T-cells from the recipient will then recognize donor tissue as “self”, and will not form an immune response against it.
In all mammalian species studied, there is a single genetic locus that encodes the strongest transplantation antigens. This is known as the major histocompatibility complex (MHC). Human leukocyte antigen (HLA) is the genetic designation for the human counterpart to the MHC. HLA restricts, and therefore regulates, the immune response in a highly specific way. Molecules encoded by HLA bind and present foreign microbial antigens to an individual's own T-cells. In fact, foreign antigens can only be recognized by the T-cells of an individual if they are presented in the context of the individual's own HLA molecules. Therefore, HLA molecules participate in a trimolecular complex comprised of the T-cell receptor, the foreign microbial antigen, and the self-HLA molecule.
In the context of transplants, donor tissue expressing non-self HLA antigens is reliably killed by cytotoxic T-cells. Tissue grafts can be rejected on the basis of HLA molecules that differ from self-HLA molecules by as little as one amino acid. It is believed that the main reason for such rejection is that non-self HLA molecules recognize and form complexes with many peptides not recognized by self-HLA molecules. T-cells are intolerant of these non-self HLA-peptide complexes, and possibly bind to distinctive features of the non-self HLA molecule.
Therefore, matching donor and recipient tissue for HLA reduces the chances of a cytotoxic T-cell response in the recipient, and thus raises the chances of survival of a transplant. However, matching itself presents a problem for transplantation. First, there are two classes of HLA molecules, class I and II. Second, there are several genes for each HLA class, and each of these genes has many alleles (i.e. HLA is polymorphic). While HLA provides an individual with the capability of a diverse immune response, it makes transplantation from one individual to another a difficult task because it makes donor-recipient matching difficult to achieve.
In humans, there are three genes for class I HLA molecules, HLA-A, HLA-B, and HLA-Cw, and three genes for class II HLA molecules, HLA-DR, -DP, and -DQ. Furthermore, HLA genes are polymorphic. There are 22 different HLA-A alleles, 42 different -B alleles, 9 different -Cw alleles, and 18 different -DR alleles. Adding to the complexity of matching, an individual has two of each A, B, Cw, and DR alleles, where one set of A, B, Cw, and DR (a haplotype) is inherited from each parent, therefore individuals may be homozygyous or heterozygous for the A, B, Cw, and DR haplotypes.
Currently, transplantation using HLA-A, -B, -Cw, and -DR matched unrelated donor tissue makes enormous demands on tissue typing resources. This is largely due to the polymorphic nature of the HLA genes, and the need for high stringency of matching to minimize rejection and acute graft vs. host disease. On average, due to HLA polymorphism, the chance of finding a donor-recipient match for HLA-A, -B, -Cw, and -DR would range from one in 1,000 to one in several million depending on the frequency of the patient's tissue type in the general population. See also, Hansen, J. A., Anasetti, C., Peterdorf, E., Clift, R. A. and Martin, P. J., “Marrow tranplants from unrelated donors,” Transplant Proc., 1994 June 26:1719–1712 (Review).
The present invention provides a reliable source of cells or tissue carrying two sets of identical HLA haplotypes, that significantly increases the possibility of a histological match between a donor and recipient, and hence, may be used for diagnosis, transplant and/or treatment. HS cells are homozygous and express only one HLA haplotype. Thus, a donor HS cell has only one HLA haplotype to be matched with the recipient instead of two, as in heterozygous stem cells. HS cells can be taken from random donors of multiple ethnicities to provide a depository or bank of HS cell populations that immunohistologically match at least one haplotype carried by individuals in the population at large. The depository needs only contain about 200 different HS cell lines, each having a distinct HLA haplotype, to service most of the general population (i.e., provide immunohistocompatible cells suitable for transplant).
Further, gene therapy in humans raises ethical concerns. A distinction has been drawn between genetic manipulations that involve only somatic cells and manipulations that may involve germ cells. Somatic manipulations are generally allowed as they affect only the patient, whereas germ cell manipulations are disallowed as they affect the patient's progeny. Moreover, research in transgenic animals has demonstrated that gene defects can be effectively corrected only by genetic manipulations of embryos. However, this approach is not feasible for gene therapy in humans for practical reasons, for example the level of insertional mutagenesis caused by the integration of retroviral vectors into cellular DNA may be unacceptably high.
The present invention also promises significant advances in gene-replacement therapy. Currently, there are many obstacles to gene therapy. Concerns such as whether a disease is suitable for genetic intervention, whether the gene for the disorder has been identified and cloned, whether there is an efficient way of introducing a gene into cells, or whether the gene can be expressed in tissues other than the affected tissue to be efficacious, significantly impede the practicability of gene therapy.
Thus, there is clearly a need in the art for a reliable source of histocompatible cells or tissue, which may be used for transplant and/or treatment, including gene therapy. For example, HS cells that are histocompatible with a patient suffering from a genetic disease can be genetically manipulated before being differentiated into a particular cell type for transplant.